The present invention relates to dual chamber or multi-chamber implantable cardiac stimulation devices. More specifically, the present invention relates to an implantable cardiac stimulation device with which a coronary sinus lead is used to stimulate the left heart chambers and in which a method is incorporated for automatically selecting left atrial and left ventricle stimulation electrode configurations such that the risk of cross-chamber stimulation is minimized.
The technology of cardiac pacemakers has become highly sophisticated with increased features, programmability, and automatization. The current generation of cardiac pacemakers incorporates microprocessors and related circuitry to sense and stimulate heart activity under a variety of physiological conditions. These pacemakers may be programmed to control the heart in correcting or compensating for various heart abnormalities that may be encountered in individual patients. It is a primary goal of programmable, multiple-mode, demand-type cardiac pacemakers to accommodate the changing requirements of a diseased or malfunctioning heart.
For example, proper synchronization of the atrial and ventricular contractions can be important in achieving normal cardiac output. In some patients, this synchrony is disrupted when the normal conduction pathway between the atria and ventricles becomes blocked due to disease. Dual chamber pacemakers are commonly used to treat such complete or intermittent heart block by maintaining atrio-ventricular (AV) synchrony. Atrial activity is monitored for evidence of a P-wave, which is the electrical depolarization occurring in the atrial myocardium producing atrial contraction. Ventricular activity is monitored for evidence of an R-wave, which is the electrical depolarization occurring in the ventricular myocardium producing ventricular contraction. In a normal heart, the electrical impulse producing the intrinsic P-wave is conducted to the ventricles with an inherent delay such that the resulting R-wave causes the ventricles to contract just after the atria have contracted. Effective cardiac output is thus achieved by allowing the atria to contract first, filling the ventricles with blood, followed by the more powerful contraction of the ventricles, which eject blood throughout the body. If the normal conduction pathways responsible for triggering and synchronizing these contractions are blocked, a dual-chamber pacemaker, upon sensing the absence of a P-wave or R-wave or both, will deliver a stimulation pulse to the atrium or ventricle or both. The ventricular pulse is delivered after a given time interval, called the xe2x80x9cAV delay,xe2x80x9d following the atrial P-wave (or atrial stimulation pulse) in order to maintain proper atrial-ventricular synchrony.
The stimulating pulse must be of sufficient energy to cause depolarization of the cardiac tissue and the subsequent contraction of the stimulated heart chamber. This condition is known as xe2x80x9ccapture.xe2x80x9d The lowest stimulation energy required to achieve capture is referred to as the capture xe2x80x9cthreshold.xe2x80x9d
Most dual-chamber (or DDD) stimulation systems utilize two leads, one connected to the atrial chamber and one connected to the ventricular chamber, to provide electrical contact for stimulating and sensing in both chambers. Each lead is connected to the appropriate channel, atrial or ventricle, of the cardiac stimulation system.
Programmability has been incorporated into cardiac stimulation devices to select the polarity of the electrodes to be used in sensing and stimulation. A unipolar lead is one in which stimulation occurs between the cathode tip and the device housing, also referred to as a xe2x80x9ccase electrodexe2x80x9d, that serves as the anode. A bipolar lead is one in which stimulation occurs between the cathode tip, and an anode ring electrode spaced approximately one to two centimeters from the cathode tip.
Unipolar or bipolar leads implanted in conjunction with conventional stimulation systems can be anchored in such a way that the tip electrodes can be positioned in contact with the targeted cardiac tissue. Intravenous leads can be advanced into the right chambers of the heart to position electrodes for stimulation and sensing in the right ventricle. The lead can be anchored in the thick muscular wall near the ventricular apex. Since the anatomical structure of the atria makes it more difficult to anchor a lead in the atrial wall, active fixation, such as a xe2x80x9cscrew-inxe2x80x9d type lead, is often used. In some cases, xe2x80x9cfloatingxe2x80x9d multi-electrode lead bodies have been designed in a way that a single unipolar electrode or a bipolar pair of electrodes on the lead body remain within the blood volume of the right atrial chamber for sensing and stimulation in the atrium. Electrodes near the distal tip of the lead make contact with the ventricular tissue. Reference is made to U.S. Pat. No. 5,999,853 to Stoop et al.
There are numerous advantages for using a single, multi-electrode lead rather than two separate leads. Lower cost, shorter implantation time, and reduced complexity and cost of a stimulation system requiring only one lead connection are all benefits associated with a single lead.
For delivering stimulation therapies in the left heart, coronary sinus leads have been developed that may be advanced through the right atrium, the coronary sinus and into the coronary veins to access the left atrium (LA) and left ventricle (LV). Refer to U.S. Pat. No. 5,466,254 to Helland which is incorporated herein by reference. By advancing a multi-electrode coronary sinus lead, dual chamber stimulation and sensing as well as cardioversion therapy may be delivered in the left heart chambers with placement of only a single lead. While this has many advantages, it is known that coronary sinus leads can be difficult to place. Care must be taken to advance the lead such that the final electrode positions result in acceptable capture thresholds. Coronary sinus leads can shift from their original locations after implantation with each shift potentially causing small but clinically significant changes in capture thresholds or in sensing of cardiac signals. An increase in capture threshold may result in loss of capture, an undesirable situation.
Another problem encountered in using coronary sinus leads is xe2x80x9ccross-chamber stimulation.xe2x80x9d Cross-chamber stimulation occurs when the stimulation pulse energy applied to capture one chamber of the heart, for example the left ventricle, is high enough to capture a second chamber of the heart, for example the left atrium. Simultaneous capture of the atrium and the ventricle is highly undesirable in that it will cause the chambers to contract against each other leading to severe cardiac output perturbation. The common practice of applying a working margin to the programmed stimulation energy to allow for fluctuation in capture threshold can inadvertently cause the stimulating energy to be high enough for cross-chamber capture.
In U.S. Pat. No. 5,265,601 to Mehra, a method for dual chamber cardiac stimulation and sensing using a single lead is proposed. The atrium is paced using a stimulation energy that is below the capture threshold for the ventricle but still high enough to capture the atrium. Since atrial and ventricular capture thresholds can be similar, the ventricular stimulation pulse is delivered following a short atrial-ventricular (AV) delay in order to avoid atrial capture. In this way, the ventricular pulse is delivered during the physiological absolute refractory period of the atria making inadvertent atrial cross-capture impossible. The limitation to this approach is a limited atrial-ventricular (AV) delay. The AV delay must be kept short enough to ensure ventricular stimulation occurs within the atrial refractory period. Such a short AV interval may not be the most hemodynamically effective and may not allow time for naturally occurring R-waves to be conducted. Inadvertent cross-capture of the ventricle during atrial stimulation is not necessarily avoided by this approach.
Ideally, stimulation of the left atrium and the left ventricle using a multi-polar lead positioned in the coronary sinus region would allow electrode polarities to be selected such that an electrode pair giving the lowest left atrial threshold and highest left ventricular threshold is selected for left atrial stimulation. Likewise, an electrode pair giving the lowest left ventricular threshold and highest left atrial threshold is selected for left ventricular stimulation. In this way, the risk of cross-chamber stimulation is minimized, and battery current drain is minimized. However, testing for these optimal electrode combinations can be time-consuming, and these optimal combinations may constantly change over time with either shifts of the coronary sinus lead location or other physiological factors.
It would thus be desirable to provide an implantable dual chamber or multi-chamber cardiac stimulation system possessing means for automatically measuring capture thresholds in each cardiac chamber using all available electrode combinations from one or more multi-polar leads. Further, it would be desirable to determine the likelihood of cross-chamber capture by measuring the threshold for cross-chamber capture from the electrode pairs available. It would also be desirable to provide a stimulation system that, based on these threshold measurements, automatically selects the optimal electrode configuration for stimulating in each chamber.
The present invention addresses these problems and more by providing an implantable cardiac stimulation device in which cross-chamber stimulation is avoided. The present invention is intended for use an implantable device that is intended to stimulate one or more cardiac chambers, for example, in a dual-chamber or multi-chamber device. According to one illustrative embodiment of the invention, capture thresholds are determined for a first chamber that is intended to be captured, and also for a second chamber that is not intended to be captured. The stimulation energy is then set to a level to capture the first chamber and to not capture the second chamber.
In one embodiment, the present invention includes a cardiac stimulation system capable of automatically determining which electrodes on a multi-polar coronary sinus lead can be used to safely stimulate the left atrium and/or the left ventricle separately without stimulating both the left atrium and the left ventricle at the same time.
The foregoing and other features of the present invention are realized by providing an implantable cardiac stimulation device equipped with cardiac data acquisition capabilities. A preferred embodiment of the stimulation device includes a control system for controlling the operation of the device and executing various test algorithms including capture threshold tests and cross-capture threshold tests; a set of leads including at least a multi-polar coronary sinus lead for receiving cardiac signals from the left atrium and left ventricle and delivering stimulation pulses in the left atrium and left ventricle; a set of sensing circuits comprised of sense amplifiers for sensing and amplifying the cardiac signals; and a set of pulse generators for generating atrial and ventricular stimulation pulses. In addition, the device includes memory for storing operational parameters for the control system and storing data such as a capture threshold test results. The device also includes a telemetry circuit for communicating with an external programmer.
In a preferred embodiment, cross-chamber stimulation is avoided by the present invention by performing the following three operations automatically: 1) determining the left atrial and left ventricular capture thresholds using all electrode combinations available on one or more multi-polar leads positioned in the region of the coronary sinus; 2) using these same electrode combinations, determine the cross-chamber capture thresholds beginning with the electrode combinations resulting in the lowest left atrial and left ventricular capture thresholds; and 3) selecting the optimal electrode configurations for left atrial and left ventricular stimulation based on these testing results. Preferably the electrode configuration selected for left atrial stimulation is one resulting in the lowest atrial capture threshold and a cross-capture threshold greater than the atrial capture threshold plus a working margin. Likewise, the electrode configuration selected for left ventricular stimulation is one resulting in the lowest ventricular capture threshold and a cross-capture threshold greater than the ventricular capture threshold plus a working margin.
Thus, safe effective stimulation of each left heart chamber is provided in a way that ensures proper atrial-ventricular synchrony with a minimal risk of cross-chamber capture and uses the minimum battery energy required.